During the first meiotic division, homologous chromosomes linked by chiasmata attach to microtubules from opposite poles of the spindle (bi-orient) and then segregate. Our long term goal is, using Drosophila melanogaster females as a model, to understand the mechanisms that promote accurate chromosome segregation on the acentrosomal spindle of oocytes. In humans, errors in chromosome segregation in the oocyte lead to aneuploidy and are the leading cause of miscarriage, infertility and birth defects. Thus, we are particularly interested in the protein complexes and mechanisms of bi-orientation and the features of the oocyte spindle that makes it susceptible to chromosome segregation errors. In the absence of the microtubule-organizing center found at mitotic spindle poles, the chromosomes generate a signal which stimulates spindle assembly. In the previous funding period, we genetically defined how two types of microtubule attachment are used for bi-orientation: first lateral attachments establish bi-orientation, then end- on attachments are required for maintenance and segregation. We found that the chromosome passenger complex (CPC) is required for both the formation of a bipolar spindle and kinetochore assembly in oocytes. The CPC localizes to the centromeres and the central spindle, which is composed of overlapping antiparallel microtubules adjacent to the chromosomes. Based on these results, the premise of the proposed studies is that bi-orientation depends on lateral interactions between the kinetochores and centrals spindle microtubules. We will test the hypothesis that SPC105R recruits additional proteins for meiosis-specific functions by determining which domains function in lateral attachments and co-orientation and identifying proteins that interact with SPC105R. To investigate the function of CPC at centromeres and central spindle during meiosis I, mutants to target the INCENP to the central spindle and centromeres will be studied. We will also test the hypothesis that bi-orientation is established by lateral attachments between kinetochores and central spindle microtubules. We will examine kinetochore-microtubule attachments in central spindle mutants, determining whether lateral and/or stable end-on attachments are formed. The Aims of this proposal are linked by a goal to understand the mechanisms of chromosome segregation that are unique to the oocyte. Upon completion of this work, we will have identified important components that have meiosis specific functions at the kinetochores, determined the role of the central spindle in bi-orientation, and characterized how this is regulated by the CPC. We will have gained insight into the mechanisms that promote bi- orientation in oocytes via the CPC, kinetochores, and central spindle. Because they are acentrosomal, there are probably segregation mechanisms that are unique to oocytes. It is important to understand these mechanisms that may make the oocyte acentrosomal spindle susceptible to certain types of chromosome segregation errors.